1. Field of the Invention
The present invention generally relates to a piezoelectric element with an electromechanical conversion function, and particularly to a piezoelectric element capable of acquiring superior piezoelectric properties upon being used in an inkjet recording head, and the manufacturing method and manufacturing device of an inkjet recording head, printer, and piezoelectric element.
2. Description of the Related Art
A typical inkjet recording head uses a piezoelectric element as the driving force to discharge ink from a printer. In general, this piezoelectric element is structured by comprising a piezoelectric thin film, and an upper electrode and lower electrode arranged so as to sandwich such piezoelectric thin film.
Conventionally, piezoelectric elements have been developed in pursuit of improving properties by defining the crystal structure of the film made from lead zirconate titanate (PZT), or forming a Ti core on the lower electrode. For example, Japanese Patent Application Laid-Open No. 10-81016 discloses a PZT thin film comprising a crystal structure of a rhombohedral crystal system as well as a prescribed orientation degree. Moreover, Japanese Patent Application Laid-Open No. 8-335676 discloses a piezoelectric element wherein a titanium core is formed on an Ir lower electrode.
Nevertheless, with conventional piezoelectric elements, there is a problem in that it is difficult to obtain a prescribed orientation degree of a piezoelectric element in a stable manner and with favorable reproducibility. With this type of piezoelectric element, it is difficult to obtain stable and superior piezoelectric properties, which becomes a contributing factor for not being able to obtain sufficient printing performance from inkjet recording heads and printers.
Meanwhile, for the manufacture of a piezoelectric element described above, a piezoelectric element with a perovskite crystal structure is formed by layering amorphous films containing metallic elements constituting the piezoelectric material, and crystallizing this by calcination at a temperature of, for example, 600xc2x0 C. with a diffusion furnace or a rapid thermal annealing (RTA) device, for example.
Pursuant to the aforementioned heat treatment, it is known that the crystal orientation of the produced piezoelectric thin film is affected by the substrate that becomes the base upon forming the piezoelectric thin film, or by the crystal direction of the lower electrode.
However, although a desired crystal orientation can be obtained in the vicinity of the lower electrode of the piezoelectric thin film with the aforementioned conventional technology, crystallinity at portions distant from the lower electrode; in other words, the upper part of the piezoelectric thin film, was apt to be poor. Particularly, when a film is formed with a thickness of 0.5 xcexcm or more, there is a problem in that the piezoelectric properties become inferior as the depreciation of the crystallinity at the upper part of the film becomes prominent.
Furthermore, there is another problem in that the displacement efficiency of the actuator becomes inferior as distortion is distributed in the film-thickness direction.
An object of the present invention is to overcome the aforementioned problems and provide a piezoelectric element comprising stable and superior piezoelectric properties by obtaining a prescribed orientation degree of a piezoelectric thin film in a stable manner and with favorable reproducibility.
Another object of the present invention is to provide a method for manufacturing a piezoelectric element with superior crystallinity and in which the crystal orientation thereof is uniform in the film-thickness direction.
A further object of the present invention is to provide an inkjet recording head employing the aforementioned piezoelectric element as the driving force for discharging ink, a manufacturing method of such inkjet recording head, and an inkjet printer.
In order to achieve the aforementioned objects, the present invention is a manufacturing method of a piezoelectric element; comprising the steps of: forming a ZrO2 film on a substrate; forming a lower electrode on the ZrO2 film; forming a Ti layer having a thickness of not less than 4 nm and not more than 6 nm on the lower electrode; forming a piezoelectric precursor film on the Ti layer; and calcinating the obtained structure;
(1) wherein the lower electrode comprises at least a first layer positioned as the uppermost layer and containing Ir and a second layer positioned as the second uppermost layer and containing Pt; the thickness of the second layer is not less than 30% and not more than 50% of the thickness of the entire lower electrode; and the step for forming the lower electrode comprises at least a step of forming the second layer containing Pt and a step of forming the first layer containing Ir on the second layer; or
(2) wherein the lower electrode comprises at least a first layer positioned as the uppermost layer and containing Pt and a second layer positioned as the second uppermost layer and containing Ir; the thickness of the first layer is not less than 20% and not more than 40% of the thickness of the entire lower electrode; and the step for forming the lower electrode comprises at least a step of forming the second layer containing Ir and a step of forming the first layer containing Pt on the second layer.
Moreover, the piezoelectric element according to the present invention comprises: a lower electrode formed on a ZrO2 film; a piezoelectric thin film formed on the lower electrode; and an upper electrode formed on the piezoelectric thin film; wherein the (100) face orientation degree of the piezoelectric thin film measured with the X-ray diffraction angle method is not less than 40% and not more than 70%.
Furthermore, preferably, the (110) face orientation degree is 10% or less, and the (111) face orientation degree is the remaining portion thereof.
FIG. 9 is a diagram showing the results upon measuring the relationship of a (100) face orientation degree of the piezoelectric thin film and the thickness of the Ti core with respect to a piezoelectric element obtained by forming a lower electrode in which an Ir layer, Pt layer, and Ir layer are laminated in that order on a ZrO2 film, forming a Ti core, and further forming a piezoelectric precursor film and crystallizing the obtained structure. In FIG. 9, symbol (a) illustrates the case where the thickness of the second layer of the lower electrode containing Pt is approximately 10% in relation to the thickness of the entire lower electrode. Here, when the thickness of the Ti core is not less than 4 nm and not more than 6 nm, the (100) face orientation degree may be enhanced to approximately 90%. If the (100) face orientation degree is adjusted to any other value than as described above, however, the variance of the (100) face orientation degree in relation to the change in thickness of the Ti core will be too large, and a desired (100) face orientation degree can not be obtained in a stable manner and with favorable reproducibility.
Contrarily, symbol (b) in FIG. 9 represents a case where the ratio of the thickness of the second layer of the lower electrode containing PT in relation to the thickness of the entire lower electrode is increased in comparison to example (a) above. Here, the (111) face orientation degree of the piezoelectric thin film rises due to the influence of Pt and, when the thickness of.the Ti core is not less than 4 nm and not more than 6 nm, the (100) face orientation degree shows a stable value.
As described above, by setting the thickness of the Ti core between 4 nm and 6 nm and adjusting the thickness ratio of the Pt layer in relation to the thickness of the entire lower electrode, the (100) face orientation degree (ratio of the (100) face orientation degree in relation to the (111) face orientation degree) of a piezoelectric thin film may be set to a prescribed ratio with favorable reproducibility.
Thus, according to the present invention, it is possible to obtain a (100) face orientation degree of a piezoelectric thin film in a stable manner and with favorable reproducibility. In addition, the ratio of the (100) face orientation degree in relation to the (111) face orientation degree may also be obtained with favorable reproducibility. It is therefore possible to provide a piezoelectric element comprising stable and high piezoelectric properties in both high frequencies and low frequencies.
Furthermore, the present invention is a manufacturing method of a piezoelectric element; comprising the steps of: forming on a lower electrode an amorphous film containing metallic elements and oxygen elements constituting a piezoelectric material; and forming a piezoelectric thin film crystallized by subjecting the amorphous film to a heat treatment; wherein during the heat treatment, heat is applied from the lower electrode side.
During the aforementioned heat treatment, the thermal energy supplied from the lower electrode side to the amorphous film is greater than the thermal energy supplied from the opposite side of the lower electrode.
The present invention also relates to a heating device realizing the heating methods described above.
Another embodiment of the piezoelectric element according to the present invention comprises a PZT thin film, and the (100) face half value breadth of the PZT thin film measured with the X-ray diffraction angle method is 0.2 degrees or less.
According to the present invention, as the substrate and heating element are arranged such that the radiant heat of the heating device is applied to the lower electrode side when heat treating and crystallizing an amorphous film containing metallic elements and oxygen elements constituting a piezoelectric material, heat is applied from the lower electrode side, and the crystal growth of the piezoelectric thin film begins from the lower electrode side and reaches the upper part of the film. It is thereby possible to manufacture a piezoelectric element with favorable piezoelectric properties as it possesses superior crystallinity and the crystal orientation is uniform in the film-thickness direction.
An inkjet recording head employing the aforementioned piezoelectric element or a printer utilizing such inkjet recording head is capable of achieving superior printing quality.